2 or 2 and a half feet sounds about right, but by the time they're that tall, they may also be a lot wider than what you're probably building. Try to build something that you can move or adapt to sprawling vines.

I grow mine under low tunnels. As the others have mentioned though, it takes a good deal of manipulation to keep them contained. The winter squash I grow isn't too bad to train, but the vining zuke is a bear. It's very turgid, and you have to move the entire vine or it will snap off.

This year I didn't get the tunnels up fast enough and the cuke beetles got a foothold. They were early. Then, to add insult to injury, the squash bugs got to the open cukes, then while hand-pollinating, they got under the squash tunnels. This was the worst year for curcurbit bugs I've ever seen. Long story short... tunnels keep the SVB out, but if the others get under there, fighting them (at least without chemicals) is an uphill battle.

How tall are your tunnels?

Probably just under 3'. I use 9' lengths of pvc pushed into the ground about 6" on either side. Were I to ever get more pipes, I would like to go higher, but as it is, the squash leaves push out the row covers, and at 118", the row cover is barely wide enough.

So, instead of using longer pipes, I'm going to try something else. I'm going to lessen the width of the beds from 4', to 3' - 3 1/2'. That way the leaves wont push out as much at the edges. I'll also change my spacing pattern to help keep the squash contained. I'll then be able to go higher by shortening the width of the poles by the 6" - 1'. I'll have fewer vines under each tunnel, but sometimes less is more.